Liquid filled display apparatus

ABSTRACT

A sign of the type having a back plate with interlaced stripes of two colors and a front plate with interlaced stripes which are alternately transparent and one of the two colors, the stripes of the front plate being selectively interrupted by sections of the other color and transparent segments to form a design, wherein the plates are in, or part of, a liquid-tight container which is filled, at least over the back plate and between the plates, with a liquid. An oily liquid which does not adversely affect any of the other materials is used. In one embodiment the stripes are horizontal and the back plate is lifted one stripe width by electromagnets which attract a ferrous bracket adhered to the plate. In another embodiment the stripes are vertical and the back plate is suspended from the top wall on nonvertical wires. The electromagnets are then mounted on a side wall.

0 United States Patent 1 l 11 3,841,010

Ludwig 1' Oct. 15, 1974 LIQUID FILLED DISPLAY APPARATUS [75] Inventor: Vernon A. Ludwig, Edmonton, 'm Exam'7erHarland skogqust Alberta Canada Asszstant Exammer-Wenceslao J. Contreras Attorney, Agent, or FirmRoylance, Abrams, Berdo [73] Assignee: Valmat Signs & Signals Ltd., & K

Alberta, Canada [22] Filed: Nov. 28, 1973 [57] ABSTRACT [21] Appl. No: 419,901 A sign of the type having a back plate with interlaced stripes of two colors and a front plate with interlaced [5'2] U 5 Cl 40/137 40/326 40/65 stripes which are alternately transparent and one of 40/52 the two colors, the stripes of the front plate being se- [51] Int Cl 609i 7/00 lectively interrupted by sections of the other color and [58] Fie'ld 37 65 62 transparent segments to form a design, wherein the 1 2 i plates are in, or part of, a liquid-tight container which is filled, at least over the back plate and between the [56] References Cited plates, with a liquid. An oily liquid which does not adversely affect any of the other materials is used. In one UNITED STATES PATENTS embodiment the stripes are horizontal and the back 957,1 l9 5/1910 Spiegel 40/137 plate is 0H6 tripe electromagnets 382,328 11/1930 wearham r 40/37 X which attract a ferrous bracket adhered to the plate. S if 5 2 In another embodiment the stripes are vertical and the 3484969 2,1969 s g a 37 X back plate is suspended from the top wall on nonvertical wires. The electromagnets are then mounted on a FOREIGN PATENTS OR APPLICATIONS Side 1 28,321 9/1931 Australia 40/326 596,261 4/1960 Canada 40 137 15 Clams 10 DrawmgFlgureS PAIimmcr 151974 SHEET 10F 3 PATENIED 3.841.010

sum 30F 3 F/G. 7 F76. 6

LIQUID FILLED DISPLAY APPARATUS This invention relates to changeable exhibitors and, particularly, to exhibitors having interlaced stripes and improved optical characteristics.

It has been known for some time that two parallel planar or similarly curved members can be produced having transparent and striped portions, generally in two colors, so that when one member is moved relative to the other, a design appears and disappears. A basic example of such an apparatus is illustrated and described in Canadian patent No. 596,261, issued Apr. 12, 1960.

A particular advantage of changeable signs of this type is that movement of only the width of one stripe (assuming that all stripes are equal in width) changes the image from a visible one to one which is substantially obscured. Thus, the technique has wide application and can be employed with a wide variety of material.

A disadvantage of the exhibitor is that some spacing is necessary between the components. Thus, in a practical embodiment employing the technique it is possible for the viewer to move to an angle such that, in the obscured position of the movable member, some portion of the image is visible. This effect, which can be referred to as a parallax effect, detracts from the otherwise dramatic and rapid change of the sign from a clear image to a simply striped pattern. Similarly, light striking at a critical angle detracts in much the same manner as the parallax effect.

An object of the present invention is to provide a changeable sign of the interlaced stripe type wherein the optical characteristics are improved to reduce the visibility of the image in the obscured position.

A further object is to provide a changeable sign in which the visual effect of the change from a totally obscured to a clearly visible design is enhanced.

Another object is to provide a changeable sign in which two parallel plates are separated by a liquid layer to facilitate relative movement thereof.

Still another object is to provide a sign in which a movable member is immersed in a liquid which provides a buoyant force to faciliate movement thereof, the liquid also providing cooling for electromagnetic motion inducing means.

Briefly described, the invention includes changeable exhibitor members of the type having a back member with alternating stripes of two different colors or surface appearances and a front member having a basic pattern of interlaced transparent and opaque stripes, the stripes being of the same width as the stripes on the back member and the opaque portions being the same color or surface appearance as one of the stripes of the back member. The front member also includes a pat tern which can be any kind of design but which commonly constitutes alphanumeric characters, the design being arranged on the front member by making selected portions of the transparent stripes opaque an selected portions of theopaque stripes transparent. The transparent portions which are made opaque are so made by using the second color from the back member,

different from the stripes in the front member. These members are contained within a housing capable of suitably supporting the members and means for producing relative motion between them. Also included within the housing is means for altering the optical characteristics of the combination, such means constituting a fluid or fluid-like substance which occupies at least the space between the two members of the changeable sign. The means for moving the members includes at least one electromagnetic coil and core structure and a ferromagnetic member on a movable one of the front and back members so that energization of the coil causes the necessary relative movement.

In order that the manner in which the foregoing and other objects are obtained in accordance with the invention can be understood in detail, particularly advantageous embodiments thereof will be described with reference to the accompanying drawings, which form a part of this Specification and wherein:

FIG. 1 is a front elevation of a portion of one of the two members constituting the basic changeable sign structure;

FIG. 2 is a side elevation, in section, of the member of FIG. 1;

FIG. 3 is a front elevation of the back member of the changeable sign structure;

FIG. 4 is a side elevation, in section, of the member of FIG. 3;

FIG. 5 is a fragmentary side elevation, in section, of an assembly according to the invention and including the members of FIGS. 1-4;

FIG. 6 is a plan view, in section, of a portion of the structure of FIG. 5 along lines 66;

FIG. 7 is a fragmentary front elevation of a further embodiment of a changeable sign assembly according to the invention:

FIG. 8 is a side elevation, in partial section, along lines 88 of FIG. 7i and FIGS. 9 and 10 are side elevations, partially broken away, showing a further embodiment of the invention.

Turning now to the drawings in detail, it will be seen that the front member of a changeable sign structure is indicated generally at 10, this member constituting a flat sheet 11 of transparent material such as glass, the sheet having sufficient thickness to be relatively stiff and rigid. On the back surface 12 of member 11 is painted, printed or otherwise affixed a pattern forming opaque and transparent portions. Also affixed to the back surface of plate 11 are guides 13 and 14 which cooperate with the side edges of the back plate, as will be furtherdescribed. Similar guides exist on the oppositeend of member 10, not shown.

As shown in FIG. 1, the pattern on the front member includes a relatively wide border 15 of one color, ren dering that portion opaque. For purposes of discussion, this color will be assumed to be white and will be referred to as such. Within the border is a'series of interlaced white stripes 16 and transparent portions 17, the white stripes and transparent portions being of equal and uniform widths. Interrupting the stripes and trans parent portions in selected areas to form the letter P are transparent and opaque portions, the opaque portions being of a different color which will be regarded as red for purposes of discussion. The red portions, shown as stippled areas 18 in FIG. 1, interrupt the transparent stripes within the outlines of the letter formed. Those portions which would otherwise be white stripes, but which lie within the confines of the letter, are transparent, these regions being identified at 19.

It will be observed that each stripe, whether transparent, white or red, is of a predetermined width a and the total width of the striped portion of the pattern on member 11 can be regarded as the dimension .t which is equal to an integral number of stripes. This is significant when considering the back member, to be discussed.

The front member also carries a motor constituting an electromagnetic coil and a ferromagnetic core 21 which is in the shape of shallow inverted U having pole pieces extending beyond the coil in the direction of the pattern and terminating in pole faces 22.

Turning now to FIGS. 3 and 4, it will be seen that the back member 25 constitutes a plate of material which can be of the same material as the front plate but which need not be transparent. The front surface 26 is substantially covered with horizontal stripes of interlaced colors, there being alternating stripes 27 of red and 28 of white. The stripes are uniform in width and are of the same widths a as the stripes in the pattern on surface 12. The overall height of the striped portion of member 25 is indicated at being equal to or greater than x+a, i.e., equal to the height of the striped portion of the pattern on plate 11 plus the width of at least one stripe. This permits the back member to be moved vertically with respect to the front member a distance equal to the dimension a without uncovering any single transparent portion of the front plate pattern.

As best seen in FIG. 4, a ferromagnetic member 30 is adhered to the back surface at the upper edge of plate 25, member 30 being disposed to be attracted by the magnetic field established at pole faces 22 when coil 20 is energized. With the coil deenergized, member 25 can be supported on a bottom bracket such as shown at 31, the bracket and pole faces 22 constituting the upper and lower limits of motion of the back plate. Either the pole faces or member 30 can be provided with a thin layer of a nonferrous material, not shown, to avoid the possibility of the freezing" effect due to residual magnetism.

It will be seen that when the back plate is behind the front plate and when, for example, stripe 33 of the back plate is aligned with transparent stripe 34 of the front plate, the top portion of the letter will be masked by the existence of a stripe of the same color behind it. However, if stripe 33 is moved so that it is aligned with white stripe 35 of the front plate, white stripe 36 of the back plate will provide a contrast for the red portion at the top of the letter, permitting that portion to be readily seen. Meanwhile, the transparent portion immediately below the top red portion of the letter will be filled by red stripe 37, thereby giving the impression ofa continuous red region for the top three stripes of the letter. The next transparent portion of the letter is then filled by the red portion of the next red stripe on the back plate, and so forth, continuing the appearance ofa continuous red region throughout the letter and continuous white regions surrounding it.

An assembled structure employing the principal illustrated in FIGS. 1-4 and incorporating the improvements of the present invention is illustrated in FIGS. 5 and 6. As shown therein, front plate 11 constitutes one major surface of a rectangular box which contains coil 20, poles 22 and back plate 25 with its ferromagnetic member 30. As seen therein, the housing also includes a rear wall 40, a bottom wall 41, a top wall 42 and side walls 43, only one of which is visible. A front gasket 45 and a rear gasket 46 are provided to seal the joints between the front and rear walls and the side walls to form a fluid tight structure. In addition, an enclosing frame structure 47 can be provided around the entire structure, leaving the front face open, to improve the fluid retaining characteristics. If necessary, spacers can be mounted on the interior of wall 40 to keep plate 25 close to plate 11.

A threaded plug 48 is provided in an internally threaded opening through upper wall 42 to permit the insertion of a fluid material which, as illustrated at 49, fills the interior of the housing at least to a level above back plate 25 and, particularly, occupies the space 44 between plates 11 and 25. Electrical conductors 50 also extend from coil 20 to a source of electrical power, not shown.

As previously described, with the system deenergized, plate 25 rests on bracket 31 as shown in phantom in FIG. 5. The plate is maintained in the proper lateral position by guides such as 13 and 14 (FIG. 1) which are disposed to be close to the edges of plate 25. The guides are preferably made of a low coefficient of friction plastic, such as nylon. Energization of coil 20 produces a magnetic field which draws member 30 upwardly to pole faces 22 thereby drawing plate 25 upwardly a distance a equal to the width of a stripe in the pattern. This produces the change described with reference to FIGS. 1 and 3 and renders the symbol visible. It will be recognized that the opposite result can occur, i.e., the symbol can be visible in the rest state and rendered invisible when coil 20 is energized.

A further embodiment of an apparatus incorporating the present invention is illustrated in FIGS. 7 and 8. As shown therein, the assembled structure includes a front plate having a pattern on its rear surface, and a back plate 56, which is movable laterally with respect to the front plate, and having a striped pattern on its front surface. Plate 55 also forms the front wall of a fluidtight housing having a rear wall 57, a top wall 58, a bottom wall 59 and side walls 60 and 61.

The patterns and the basic technique for rendering the pattern visible and invisible are fundamentally similar in FIGS. 7 and 8 to that shown in FIGS. 1-6 and no detailed discussion thereof will be undertaken. It will be observed that the stripes are vertical rather than horizontal and that the motion of the back plate is therefore necessarily horizontal rather than vertical. As shown in FIG. 7, the back plate is in the position which permits the symbol to be most visible and the details of the stripes on the front plates have therefore been omitted. They would, in fact, be substantially invisible under the circumstances shown. Likewise, only the stippling indicating the color red is shown in FIG. 7 for simplicity. Still further, gaskets and other such details have been omitted to simplify the figure.

As shown, the apparatus further comprises coils 62 and 63 which are electrically connected in parallel circuit relationship to an energizing circuit including a source of voltage indicated as a battery 64 which is connected in series circuit relationship with a switch indicated generally at 65. For repetitive periodic operation, switch 65 can be operated by a cam 66 continuously driven by a motor 67. This will repeatedly energize the coils 62 and 63.

Coils 62 and 63 are provided with pole members 68 and 69, respectively, which members terminate in pairs of pole faces parallel to the edge of movable plate 56.

Plate 56 is provided with magnetizable members and 71 which are adhered to the back of plate 56 in a position to be attracted by the field established between the pole faces when coils 62 and 63 are energized, thereby attracting the plate to the pole faces.

At the lower edge of plate 56 are rectangular blocks 75 and 76 which are adhered to the back of the plate for support purposes. Body 76 also includes a portion which protrudes beyond the edge of plate 56 opposite ferrous members 70 and 71, the protrusion being for the purpose of establishing a limit of motion of plate 56 away from the electromagnetic poles. A similar spacer block 77 is attached to the upper corner of plate 56 farthest from the electromagnetic motors, this member protruding beyond the plate by the same distance as member 76. These members abut side wall 61 when coils 62 and 63 are deenergized, the protrusion being selected to limit the lateral motion of plate 56 to a distance b which is equal to the width b of the stripes on the back plate pattern.

Plate 56 is suspended by a wire arrangement including wires 80 and 81. Wire 81 is essentially straight for most of its length and terminates at its lower end in a U-shaped book 82, the distal end of which protrudes upwardly and is received in a hole which extends upwardly from the lower surface of block 75. The upper end of wire 81 is bent into a downwardly opening hook 83 which is received in an arcuate recess into the lower surface of upper wall 58. Hook 83 partially surrounds a pin 85 which supports the hook and wire 81.

In similar fashion the lower end of wire 80 is formed into an upwardly opening hook 86 which is received by an opening in the lower surface of body 76. The upper end of wire 80 is bent into a downwardly opening hook 87 which is received in an arcuate recess 88 in upper wall 58, the hook, wire and plate being supported by a pin 89.

It will be observed that the openings into which books 86 and 82 are received are laterally offset from the respective arcuate recesses in which the upper ends of the wires are received and the pins from which they are suspended. Thus, when the apparatus is in its upright position, the force of gravity tends to move plate 56 away from the electromagnetic motors in a swinging motion. The motion is, for all practical purposes, linearly horizontal because of the very small extent of motion necessary.

It will also be observed that wall 58 of the housing includes a threaded plug 90 by which fluid can be introduced into the housing.

The introduction of fluid into the housing of both of the foregoing embodiments disclosed has a beneficial effect on the optical properties thereof as suggested earlier. In any devices of this type, it is desirable to place the patterns on the front and back plates on the surfaces which are closest to each other, i.e., on the back of the front plate and on the front of the back plate. However, this does not totally eliminate the parallax effect because of the spacing desired. Providing fluid between these plates has the beneficial effect of, optically, eliminating the gap. Although there is still space between the patterns on the plates, the effect of that space is essentially eliminated and the non-aligned appearance disappears. Thus, the effect from a uniform striped pattern to the appearance of letters of another design is sudden and dramatic when the coils are energized and the movable plate is moved.

Various fluids can be used to fill the housing as de scribed herein. The specific fluid selected can depend to some degree on the specific environment inwhich the entire structure is to be used, but, generally speaking, any liquid which will not freeze under the anticipated temperature circumstances and is compatible with the various sealants, plastics, paints or inks employed in the apparatus will be suitable. Naturally, it should be clear or very nearly transparent, although a colored liquid could be employed for special effects. Oily fluids are advantageous because they reduce the effect of'friction between adjacent moving components and also minimize corrosion affects. Glycol, silicone oil and hydrocarbon oils have been employed and have been found to be quite suitable. The viscosity of the liquid can be quite high to the point of being gelatinous and it is contemplated that gelatin. should be included within the term fluid.

It will also be recognized that in either of the embodiments discussed with reference to FIGS. ll-S the coils could be mounted outside of the housing with the pole pieces protruding through a non-ferrous upper wall or through the top ofthe back wall, the opening therefor being sealed against fluid leakage. However, placing the coils within the housing permits the further advantage of using the fluid for cooling purposes. This is particularly advantageous for long, continuous cycling of the sign or where less expensive coils are to be used.

The use of a liquid filled housing has other substantial advantages, including the prevention of the formation of condensation on the moving parts of the sign and subsequent freezing thereof in cold climates, rendering the sign inoperative. Using a liquid with a low freezing point excludes freezeable liquids.

Still further, incorporation of the liquid permits a substantial saving in the energy needed to move a movable member. The buoyant effect of the liquid displaced decreases the effective weight of the movable plate, permitting the construction of larger signs with economically reasonable electromagnetic devices. Most particularly, heavy members can be most easily and effectively moved using a combination of flotation, pendulum suspension and lubrication.

The term stripes as used herein should be read to include stripes which are curved as well as straight, and it must be recognized that uniformity in width is not a requirement. However, equality of width is needed along any line in the direction of plate motion between stripes on the two plates for the effect described herein to occur.

The term opaque is used herein for simplicity, but it should be recognized that some stripes may advantageously be translucent. This would be particularly true if the housing back is transparent or translucent and the sign is to be back lighted. Thus, the stripes need only be vision obscuring.

It will also be seen that the plates need not be either flat nor rectangular, but must be similar in surface shape.

Yet another embodiment of the invention is shown in FIGS. 9 and 10, these figures being somewhat simplitied with support members and the like being omitted. This embodiment includes a front plate 101 and a back plate 102, these plates being provided with stripes and design characteristics in the same general fashion as described with reference to FIGS. 1-l8. Back plate 102 is provided with a ferrous member 163 which is magnetically attracted to pole faces of an electromagnetic structure indicated generally at 104 and including a core 105 and a coil assembly 106. As shown in FIG. 9, plates 101 and 102 are parallel to each other and are rather closely spaced, the spacing in the figure being slightly exaggerated for simplicity.

The embodiment of FIGS. 9 and 10 is also provided with an elastomeric membrane 107 which extends along the edges of plates 101 and 102 and is cemented to those edges at 108 to provide a fluid-tight seal completely around the peripheries of the two plates. This assembly forms a fluid-tight cavity 109 which can then be filled with a fluid which, as previously described with reference to the embodiments of FIGS. l-8, is consistent with the materials employed, has a suitably low freezing point, etc. The fluid can be inserted through the membrane with a hypodermic needle, for example, or through an opening in the membrane which is subsequently closed as by an additional layer of the material, not shown. Alternatively, the sides and .bottom can be covered, the cavity can be filled, and the top can then be sealed with an additional strip of the membrane.

The membrane itself should be characterized by sufficient flexibility so that plate 102 can be laterally moved with respect to plate 101 by a distance equal to the width of one stripe of the pattern. It will be observed that some motion of the plates toward each other may be necessary, but that this motion is sufficiently small so that no optical distortion occurs if the width of the stripes is chosen to be relatively small. The relationship of the members would then be as shown in FIG. 10 with plate 102 moved upwardly by the attraction of magnetic member 103 to the pole faces of electromagnet 104.

Plate 101 can constitute the front plate of an assembly which would further comprise structural members to maintain the electromagnet in appropriate structural relationship with the plates. This framework can take any convenient form and will therefore not be described in greater detail.

While certain advantageous embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

1. In a changeable sign of the type having first and second relatively movable members in which the first member is provided on one surface with a pattern of interlaced transparent and vision obscuring stripes interrupted by a design having vision obscuring and transparent portions, respectively, and in which the second member is provided on one surface with a uniform pattern of two different colors of interlaced vision obscuring stripes, the improvement comprising housing means for supporting the second member adjacent to the first member with the patterned surfaces in closely spaced relationship and with the other surface of said first member exposed to view,

said housing means comprising means for enclosing a volume to form a fluidtight container,

the patterned surface of said second member being contained within said volume;

a fluid in said volume including the region between said first and second members; and

means for causing relative movement between said first member and said second member between a first position in which the design on said first member is rendered visible and a second position in which the design is not visible.

2. A sign according to claim 1 wherein said first member constitutes one wall of said means for enclosing a volume.

3. A sign according to claim 2 wherein said means for causing movement includes an electrical coil;

a core passing through said coil and terminating'in pole faces;

means for mounting said coil and core on said means for enclosing a volume with said pole faces directed toward an edge of said second member;

a magnetizable member secured to said second member in a position to be attracted by a magnetic field generated between said pole faces; and

means for selectively energizing said coil to produce a magnetic field between said faces.

4. A sign according to claim 3 wherein the striped patterns on said first and second members are generally horizontal and said coil and core are mounted at the top of said volume, in its customary orientation, to lift said second member from a first position to a second position when said coil is energized,

said second member being returned to its lower position by gravity upon deenergization of said coil.

5. A sign according to claim 4 wherein said first member further comprises stop means below the lower limit of said pattern for supporting and limiting the downward motion of said second member; and

guide means to prevent relative lateral motion of said second member.

6. A sign according to claim 3 wherein said means for enclosing includes an upper wall and means for suspending said second member from said upper wall.

7. A sign according to claim 6 wherein said means for enclosing comprises a side wall perpendicular to said first member,

said coil and core being mounted adjacent said side wall, and

said magnetizable member is secured to a vertical edge of said second member closest to said side wall,

the striped patterns on said first and second members are generally vertical.

8. A sign according to claim 7 wherein said second member further comprises stop means mounted on said second member for limiting the lateral movement of said second member to a distance equal to an integral number of stripes widths.

9. A sign according to claim 6 wherein said means for suspending includes first and second wires each pivotally supported at one end from said upper wall, and

means attached to said second member near the lower edge thereof for engaging the other ends of said wires.

10. A sign according to claim 6 wherein said upper pole faces; wall includes means for permitting introduction of fluid means for mounting said coil and core with said pole into said volume. faces directed toward an edge of said second mem- 11. A sign according to claim 1 wherein said fluid h Prises 5 a magnetizable member secured to said second meman oily liquid selected from the group consisting of ethylene glycol, silicone oil and hydrocarbon oil. 12. A sign according to claim 1 wherein said fluid comprises an oily liquid.

A Sign according to Claim 1 wherein Said means lo 15. A sign according to claim 14 wherein the striped for enclosing a volume comprises a flexible elastomeric membrane extending between patiems on Said first and second members are generally adjacent edges of said first and second members; horizontal and her in a position to be attracted by a magnetic field generated between said pole faces; and

means for selectively energizing said coil to produce a magnetic field between said faces.

and said coil and core are mounted at the top of said vol- I means for adhering said membrane to said members 15 Customary onent'afloni to 11ft said f to f a fl id ti h container 0nd member from a first position to a second post- 14. A sign according to claim 13 wherein when Said C011 18 glz said means for causing mov ent i lud said second member being returned to its lower posian electrical coil; tion by gravity upon deenergization of said coil.

a core passing through said coil and terminating in 20 

1. In a changeable sign of the type having first and second relatively movable members in which the first member is provided on one surface with a pattern of interlaced transparent and vision obscuring stripes interrupted by a design having vision obscuring and transparent portions, respectively, and in which the second member is provided on one surface with a uniform pattern of two different colors of interlaced vision obscuring stripes, the improvement comprising housing means for supporting the second member adjacent to the first member with the patterned surfaces in closely spaced relationship and with the other surface of said first member exposed to view, said housing means comprising means for enclosing a volume to form a fluidtight container, the patterned surface of said second member being contained within said volume; a fluid in said volume including the region between said first and second members; and means for causing relative movement between said first member and said second member between a first position in which the design on said first member is rendered visible and a second position in which the design is not visible.
 2. A sign according to claim 1 wherein said first member constitutes one wall of said means for enclosing a volume.
 3. A sign according to claim 2 wherein said means for causing movement includes an electrical coil; a core passing through said coil and terminating in pole faces; means for mounting said coil and core on said means for enclosing a volume with said pole faces directed toward an edge of said second member; a magnetizable member secured to said second member in a position to be attracted by a magnetic field generated between said pole faces; and means for selectively energizing said coil to produce a magnetic field between said faces.
 4. A sign according to claim 3 wherein the striped patterns on said first and second members are generally horizontal and said coil and core are mounted at the top of said volume, in its customary orientation, to lift said second member from a first position to a second position when said coil is energized, said second member being returned to its lower position by gravity upon deenergization of said coil.
 5. A sign according to claim 4 wherein said first member further comprises stop means below the lower limit of said pattern for supporting and limiting the downward motion of said second member; and guide means to prevent relative lateral motion of said second member.
 6. A sign according to claim 3 wherein said means for enclosing includes an upper wall and means for suspending said second member from said upper wall.
 7. A sign according to claim 6 wherein said means for enclosing comprises a side wall perpendicular to said first member, said coil and core being mounted adjacent said side wall, and said magnetizable member is secured to a vertical edge of said second member closest to said side wall, the striped patterns on said first and second members are generally vertical.
 8. A sign according to claim 7 wherein said second member further comprises stop means mounted on said second member for limiting the lateral movement of said second member to a distance equal to an integral number of stripes wiDths.
 9. A sign according to claim 6 wherein said means for suspending includes first and second wires each pivotally supported at one end from said upper wall, and means attached to said second member near the lower edge thereof for engaging the other ends of said wires.
 10. A sign according to claim 6 wherein said upper wall includes means for permitting introduction of fluid into said volume.
 11. A sign according to claim 1 wherein said fluid comprises an oily liquid selected from the group consisting of ethylene glycol, silicone oil and hydrocarbon oil.
 12. A sign according to claim 1 wherein said fluid comprises an oily liquid.
 13. A sign according to claim 1 wherein said means for enclosing a volume comprises a flexible elastomeric membrane extending between adjacent edges of said first and second members; and means for adhering said membrane to said members to form a fluid-tight container.
 14. A sign according to claim 13 wherein said means for causing movement includes an electrical coil; a core passing through said coil and terminating in pole faces; means for mounting said coil and core with said pole faces directed toward an edge of said second member; a magnetizable member secured to said second member in a position to be attracted by a magnetic field generated between said pole faces; and means for selectively energizing said coil to produce a magnetic field between said faces.
 15. A sign according to claim 14 wherein the striped patterns on said first and second members are generally horizontal and said coil and core are mounted at the top of said volume, in its customary orientation, to lift said second member from a first position to a second position when said coil is energized, said second member being returned to its lower position by gravity upon deenergization of said coil. 